Electrostatic printing using porous member



June 17, 1969 x 3,450,043

ELECTROSTATIC PRINTING USING POROUS MEMBER Filed Aug. 14. 1967 Shee t of 2 INVENTOR. JEROME FLAX BYO{ June 17, 1969 J FLAX ELECTROSTATIC PRINTING USING POROUS MEMBER Sheet Filed Aug. 14, 1967 LOA D Q POWDER POWDER LOAD E50 SOURCE LOW PRE$$URE NR $OURCE POWDER RESERVOKR BLOWN OWDER SUPPLY POWDER COLLECTOR M 5 WE I? M United States Patent 3,450,043 ELECTROSTATIC PRINTING USING POROUS MEMBER Jerome Flax, Ross, Calif., assignor, by mesne assignments,

to Monsanto Graphic Systems, Inc., St. Louis, Mo., a corporation of Delaware Filed Aug. 14, 1967, Ser. No. 660,523 Int. Cl. B411 13/00; B4113 15/00 US. Cl. 101-114 12 Claims ABSTRACT OF THE DISCLOSURE The process of electrostatic printing involves employing an electric field for moving a powder to form an image from the location at which the powder image defining structure is located, to a receiving substrate. The powder image, an accordance with this invention, is formed on the surface of a sponge-like material, which is positioned within the electric field employed for transferring the powder image.

Background of the invention Field of the inventi0n.This invention relates to electrostatic printing processes, and, more particularly, to improvements therein.

Description of the prior art.In a Patent No. 3,081,- 698, to Childress et al., there is described an electrostatic printing system wherein a screen is masked except for openings forming image-shaped regions. A powder image of these regions is formed by applying powder to one side of the screen whereby it passes through the image-shaped apertures. The screen is used as an electrode in establishing an electric field with an oppositely positioned electrode. A substrate, or image receiving object, is positioned within the electric field. If the powder particles have electroscopic properties and the polarity of the potential applied for establishing the field is proper, then the powder image is carried to the object and deposited thereon. The image adheres to the object until it can be subsequently fixed.

The patent shows different ways of applying powder to the back of the screen for forming the powder image. These include the use of rollers or brushes, and also the use of a powder cloud. Many other techniques have been developed since this patent was issued for applying powder to the screen. They all, of necessity, use the screen for establishing the powder image. While, where heavy powder deposits are permitted, the screen mesh has no aifect on the resulting print which is made, however, where the powder deposited is quite light, the screen pattern can be seen in the printing. This reduces the intensity of the print.

When powder particles are pushed through the image apertures of a screen by some mechanical force, such as that provided by a brush, while the majority of the powder particles are passed into the field with a motion perpendicular to the screen, some of the powder particles are given a motion which is somewhat transverse to this and thereby can fall outside the desired powder image which obviously is not desirable.

Accordingly, an object of this invention is the provision of an electrostatic printing system which provides a new and improved arrangement for forming the powder image.

Yet another object of the present invention is the provision of a means for forming the powder image without using the image screen.

Still another object of the present invention is the provision of a new and improved electrostatic printing system.

Summary of the invention The foregoing and other objects of the invention are achieved in an arrangement wherein a spongy porous material, such as a sheet of polyurethane foam which has a plurality of closely spaced pores opening out to the surface, has one side covered by an image mask. Electroscopic powder is applied to the regions of the sponge which are image-shaped. The powder remains in place thereafter without the application of any other forces. The spongy material carrying the powder image is then placed in an electric field. When the electric field is of the proper polarity, it moves the powder image from the sponge to an oppositely positioned object.

Brief description of the drawings FIGURE 1 illustrates an embodiment of the invention.

FIGURE 2 is an isometric drawing illustrating an arrangement of one portion of FIGURE 1.

FIGURE 3 illustrates another embodiment of the invention.

FIGURE 4 illustrates a third embodiment of the invention.

FIGURE 5 shows another form which this invention may take.

FIGURE 6 shows the invention being set up in the manner of a printing press.

FIGURE 7 shows the invention being used in a continuous system.

FIGURE 8 shows another arrangement for using the invention in a continuous system.

FIGURE 9 and FIGURE 10 illustrate the ability of this invention to be used for printing on objects having unconventional shapes.

FIGURES 11 and 12 show two other arrangements for loading powder on the embodiment of the invention.

FIGURE 13 illustrates an arrangement for compressing the powder carrying spongy material.

Description 07 the preferred embodiment of the invention FIGURE 1 is an isometric view illustrating schematical- 1y an embodiment of the invention. This comprises a conductive screen 10, which is attached to a source of field establishing potential 12. The potential source 12 is connected through a switch 14 to back plate 16. Accordingly, when the switch 14 is closed, an electric field is established between the screen 10 and the back plate 16'. A substrate 18, or document, upon which it is desired to print, is placed adjacent the back plate 16. Anywhere between the substrate 18 and the screen 10, but preferably adjacent to the screen, there is placed a sheet 20 of foam or porous material.

As shown in FIGURE 2, the foam material 20 has a mask 22 or masking material 22 on one surface thereof, with openings therein, to expose the underlying porous surface in the form of an image 24. The material may be a sheet of polyurethane, for example. The mask, which is applied to the surface of the polyurethane, may be a separate sheet of material having image openings therethrough, or may be a coating which is impervious to powder particles except in the regions defining the image desired to be printed. This may be the type of coating used in making silk screens, for example.

In order to make use of the embodiment of the invention, electroscopic powder particles are applied to the exposed image areas of the porous sheet, either by the direct application of a brush loaded with powder particles, by dipping the sheet into a powder box while a vacuum is applied to the back side of the porous sheet, or by any other suitable known technique. Any powder which remains on the mask surface is doctored off. Then, with the equipment arranged as shown in FIGURE 1, when the switch 14 is closed, the electric field which is estab- 3 lished causes the powder particles to transfer from the sheet material to the image-receiving substrate in the form of powder images. The powder particles that transfer are those which are retained in the porous openings which are exposed through the mask.

The pores in the material of which the porous sheet is made need not be uniform and need not be smaller than the size of a powder particle. They may be large enough to receive a considerable amount of powder, however the depth of the pores should not be so great that the weight of the powder collected in a pore causes it to fall out due to gravity. As a rule, the powder particles tend to agglomerate. The particles at the edge of an agglomerate adhere to the walls of a pore in the porous material. Where powder falling out of pores is a problem, the sheet of porous material may be held lower than the image receiving substrate and printing may be performed upward against the forces of gravity.

In this invention, the powder image to be transferred is positioned within an electric field and between the two electrodes which establish the field. The field causes a transfer of a powder image to the image-receiving substrate.

It should be appreciated from the foregoing that the screen, which serves the function of an electrode for establishing the electric field required for powder image transfer may actually be replaced by any electrode which establishes an electric field over the desired regions of powder image transfer. Thus, FIGURE 3 illustrates schematically an end view of an electrode 26 in the form of a metal sheet or foil which is positioned on the porous sheet 20. The other structures shown in the drawing are the same as those described in connection with FIGURE 1 and therefore will not be redescribed.

If the sheet of porous material is conductive, then no additional electrode is necessary to establish an electric field. This can be accomplished by applying a coating to the surface opposite the masked surface, of a conductive material, such as a silver paste or an aluminum paint. Alternatively, the porous sheet may be immersed in a solution which renders it conductive, such as nickel strike solution or electroless plating solutions.

FIGURE 4 shows an arrangement wherein the spongy material 30 has been rendered conductive by any one of the techniques described. The remaining structures shown in FIGURE 4 are the same as those shown in FIGURE 1 and therefore bear the same reference numerals. It should be noted that while the foregoing embodiments of the invention show a back plate 16 being employed as an electrode for terminating the electric field lines emanated from the opposite electrode, however, in accordance with the teachings of the Childress patent, previously recited, if the object which is to receive the printing is sufficiently conductive to be able to establish an electric field with the opposite electrode which is strong enough to effectuate transfer of the powder particle image, the back plate may be dispensed with, and the electrode connection may be made directly with such object.

FIGURE 5 shows an arrangement which the present invention makes possible. Instead of a sheet of porous material which is coextensive with the screen or plate electrode and which is masked except for the image apertures, the porous sheet material may be cut in the form of the desired image, such as the letters 32, 34 and 36. These may be affixed to an electrode 38 by any suitable means such as glue, wire staples, etc. Powder particles may he applied to the image-shaped porous material in any suitable manner and thereafter the electrode holding the raised images may be placed opposite another electrode and substrate for the purpose of transferring the powder images to the substrate.

It should be appreciated that the arrangement just described affords a very quick setup for electrostatic printing, if one provides in advance a library of porous material images, these may be used for setting up an electrostatic printing press, for example, in the manner that type is assembled for wet ink printing. FIGURE 6 illustrates such an arrangement. Here the porous material of which the letters 32, 34 and 36 are made is rendered conductive in any one of the manners recited above. The letters are held between two conductive rails 37, 39, which are supported on a conductive plate 41. The letters are slid between the rails to be supported thereby, and may be removed therefrom when no longer needed. Electrical connection is made either to the back plate or to the rails.

FIGURE 7 shows an embodiment of the invention which provides for continuous operation. The porous sheet material is in the form of a belt 40, which is supported on two spaced rollers 42, 44. The mask 46 is also in the form of a belt which is on the outer surface of the porous sheet material 40. The roller 44 is driven from a motor 48. As the porous sheet material and mask, which will hereafter be referred to as the powder image forming device, are rotated, it first passes a powder loading station. This powder loading station comprises a container 50 of the triboelectric pigment powder with which printing is to be performed. A rotating brush 52 picks up a supply of powder and applies it to the mask 46 and through the openings in the mask to the porous sheet material which is exposed. A doctor blade 54 which is on the trailing edge of the box, removes the excess powder from the mask. The powder image forming device next passes in the space between two electrodes 56, 58. These electrodes are connected across a source of potential 60, which thereby establishes an electric field therebetween. A web 62, on which printing is to occur is fed from a. payoff reel 64 and over two rollers 66, 68, which function to carry the web through the electric field at a speed which is matched to that of the traveling powder image forming device. This is insured by the drive from the motor 48, which drives a pickup reel 70. The powder image is caused to transfer from the powder image forming device to the web which is biased opposite to it by the electric field established between the two electrodes 56 and 58. A powder image drying station 72, which may merely consist of a heater, is positioned between the roller 68 and the pickup reel 70.

Instead of having a belt-type configuration, the powder image forming device may have a cylindrical configuration. This is shown in FIGURE 8. The cylindrical powder image forming device 74 is supported by a drum 76. This is rotated by a motor 78. The drum, in rotating, passes by a powder load source 80, which may be any of the arrangements previously described for applying powder to the outer image forming device. The drum then continues to rotate until the powder image-forming device is brought into the region where the transferring electric field is established. If the drum is made conductive, then it can be connected to a source of operating potential 82, which is also connected to an electrode 84, spaced from the powder image-forming device surface.

A first roller 86 pays out a sheet of web material 88, upon which printing is to occur. This web is carried by a pair of spaced rollers 90, 92 and back plate 84 through the electric field region, wherein the powder image is transferred to the web. The web then passes through a fixing station 94, similar to the fixing station 72. The web then passes onto a pickup reel 96.

In transferring a powder image in an electric field from whatever the mechanism for forming the powder is to the receiving object, it has been found that if the image receiving object is curved, distortion of the received powder image is avoided by, for example, curving the image apertured screen to be parallel to the curvature of the image receiving object. However, the delivery of powder to the back side of curved screen in a manner to insure a uniform distribution over the region of the image as well as a uniform thickness of the powder image, is

extremely difiicult. The present invention easily corrects this problem.

Referring now to FIGURE 9, there is shown a complex curved object 100, such as a bottle. A powder image forming device, in accordance with this invention, consisting of a porous sheet 102, is easily conformed to the curvature of the bottle 100. As a matter of fact, if polyurethane foam is employed, a bottle may be used as a mold when the polyurethane is foamed. The foam has a metal film 104, for example, on one side, and a mask 106 on the other side. A source of potential 108 connects the metal film to the bottle 100 which is sufficiently conductive to establish an electrical field with the metal film 104.

FIGURE shows an arrangement of the embodiment of the invention being used for nip printing. Here a rotating object, such as a can, 110, is positioned opposite a roller 112, over which there is moved a powder image forming device 114, in accordance with this invention. A powder loading station 116 loads powder into the image apertures of the powder image forming device prior to its being brought to the nip 16, which is the region at which the powder image forming device and the can 110 are closest to one another.

FIGURES 11 and 12 illustrate two different techniques of loading powder. The powder image forming device 120, shown in FIGURE 11, has a screen 122 at the back surface thereof. Powder is blown, under the pressure of air, from a supply source 124 at the back surface of the screen. The powder particles, if sufficiently fine, can pass through the screen openings and through the pores of the porous material. Any excess powder, which passes completely through the porous material in the region of the image apertures, is collected by an excess powder collector 126. The edges of the device may be sealed to prevent powder from being blown out sideways. The powder image forming device is moved from the powder loading station to the station, not shown, at which the powder image is transferred.

FIGURE 12 shows an arrangement for loading powder into the image regions of the powder image forming device using low air pressure. Here a powder reservoir, 130, is positioned at the mask side of the powder image forming device 120, and a low pressure air source 136 is applied to the back side of the screen 122. Here again the edges of the powder image forming devices are sealed to block a path to atmospheric pressure therethrough.

It may be desirable to mechanically eject the powder particles from the loaded foam through the image apertures and toward the receiving substrate to supplement the carrying force of the electric field. This may be done by compressing the foam at the time of printing. A simplified arrangement for doing this is shown in FIG- URE 13. Here the mask 134 is made of a rigid material. The plate 135 is made of a rigid material so that when pressure is applied thereto by means of a handle 138 attached thereto, the foam 140 is compressed against the mask which is supported on supports 142, 144. Pressure is applied at the time a switch 146 is closed to connect a potential 148 between plate 136 and the receiving substrate 150. The squeezed foam ejects powder into the field between substrate and plate.

There has, accordingly, been shown and described herein a novel, useful and improved electrostatic printing system wherein an image made of electroscopic powder particles is established by a powder image forming device within the electric field formed between two spaced electrodes which serve to transfer the powder image from the forming device to an image-receiving substrate.

I claim:

1. In an electrostatic printing system of the type, wherein an electric field is established between two oppositely spaced electrodes, for transferring electroscopic powder particles to form an image on an image-receiving object positioned within the said electric field adjacent one of said electrodes, the improvement comprising a sheet of porous material positioned within said electric field between the two oppositely spaced electrodes and adjacent the other of said electrodes, said porous material having porous passageways extending between opposite sides which are larger than particles of said powder to permit powder transfer therethrough, a mask having imageshaped apertures, said mask being positioned adjacent the surface of said porous material sheet which faces said image receiving object, and means for applying electroscopic powder to the regions of said porous material sheet which are adjacent to the apertures in said mask.

2. An electrostatic printing system comprising an electroscopic powder image forming device, an image-receiving substrate, and means for establishing an electric field between said powder image forming device and said image-receiving substrate, to transfer said powder image to said image-receiving substrate, said powder image forming device comprising a porous material with pores larger than particles of powder in said electroscopic powder image, said porous material having the form of said desired image, said means for establishing an electric field, including a first and second electrode respectively positioned .adjacent the surfaces of said powder image forming device and said image-receiving object, which do not face one another.

3. An electrostatic printing system comprising a first electrode, a second electrode spaced opposite from this first electrode, an image-receiving object placed in the space between said electrodes adjacent said second electrode, a powder image forming device positioned adjacent said first electrode and in the space between said first electrode and said image receiving object, said powder image forming device comprising a sheet of porous material having porous passageways larger than particles of powder used for said powder image extending between opposite surfaces to permit the transfer of electroscopic powder particles therebetween a mask having openings in the shape of desired images therein, said mask being positioned on the surface of said sheet of porous material which is opposite to said image receiving substrate, means for applying electroscopic powder to the regions of said porous material which are exposed through the apertures of said mask, and means for establishing an electric field between said electrodes for transferring the powder image formed in the apertures of said mask to said image receiving substrate.

4. An electrostatic printing system as recited in claim 3 wherein said porous material sheet is foamed polyurethane.

5. An electrostatic printing device as recited in claim 3 wherein said mask is made of rigid material, and there is included means to compress said porous material sheet between said first electrode and said mask.

6. An electrostatic printing system as recited in claim 3 wherein said second electrode is a conductive metal sheet.

7. An electrostatic printing system as recited in claim 3 wherein said first electrode is a conductive coating on said foamed polyurethane.

8. An electrostatic printing system comprising means for forming an electroscopic powder image including a sheet of a spongy, porous material having porous passage ways larger than the powder particles in said electroscopic powder image extending between opposite surfaces to permit the transfer of electroscopic powder particles therethrough, a mask having image apertures adjacent one surface of said sheet of said porous material, means for applying electroscopic powder particles through said porous material sheet to the region of the openings of said apertured mask, an image-receiving substrate spaced from said apertured mask, and means including the surface of said porous material opposite said one surface to which said apertured mask is applied for establishing an electric field for transferring the powder particles 7 from said spongy porous material to said image-receiving substrate.

9. An electrostatic printing system as recited in claim 8, wherein said means for establishing an electric field between said porous sheet of material and said imagereceiving substrate includes a conductive coating on the surface of said foam porous sheet of material which is opposite to the surface adjacent to which said mask is positioned, and means for applying a potential between said conductive coating and said image-receiving substrate.

10. An electrostatic printing system as recited in claim 8, wherein said means for establishing an electric field for transferring said powder image includes a conductive material deposit over said porous sheet, a source of potential and means for applying potential from said source between said foam porous sheets and said image-receiving substrate.

11. An electrostatic printing apparatus as recited in claim 8, wherein said means for establishing an electric field includes a first electrode made of a screen material and positioned adjacent the surface of said porous material sheet which is opposite to the one to which said mask is applied, and a second electrode adjacent said image-receiving substrate, a source of operating potential and means for applying potential from said source between said first and second electrodes.

12. Electrostatic printing apparatus as recited in claim 8, wherein said means for establishing an electric field comprises a first and second electrode, each made of a metal sheet, a source of operating potential, and means for applying potential from said source to said first and second metal sheets.

References Cited UNITED STATES PATENTS 2,787,556 4/1957 Haas.

3,081,698 3/1963 Childress et a1.

3,180,256 4/1965 Kramer et al 101-129 3,245,341 4/ 1966 Childress et al.

3,282,207 11/ 1966 Plymale.

3,285,167 11/1966 Childress et al.

3,285,168 11/ 1966 Childress.

3,332,344 7/ 1967 Londahl et al.

3,340,802 9/1967 Pilon.

3,352,234 11/ 1967 Londahl.

EDGAR S. BURR, Primary Examiner. 

